There is a worldwide shortage of healthy organs for transplant in general, in particular, epithelial-derived organs, such as the liver, pancreas, thyroid and pituitary. For example, a shortage of livers exists for orthotopic organ transplant, as a source of primary hepatocytes for clinical therapies to treat acute and chronic liver failure, and for extracorporeal liver assist devices. Attempts to propagate primary human hepatocytes in culture have met with limited success, because adult human hepatocytes, unlike newborn-derived hepatocytes, do not have a high proliferative capacity. One solution to meet the overwhelming demand for human cells is to employ immortalization strategies to expand the hepatocyte population. Unfortunately, proliferating human hepatocytes tend to lose critical functions such as their ability to metabolize drugs and ammonia in vitro. Another strategy is to use primary porcine hepatocytes or organs for transplants. However, recent concern for the risk of zoonotic infections, such as porcine endogenous retroviruses, may limit the use of xenotreatments in the future.
Bioresorbable scaffolds may be used as a temporary scaffolding for transplanted cells, and thereby allow the cells to secrete extracellular matrix of their own to enable, in the long term, a complete and natural tissue replacement. The macromolecular structure of these scaffolds is selected so that they are completely degradable and are eliminated, once they have achieved their function of providing the initial artificial support for the newly transplanted cells. For these scaffolds to be useful in cell transplantations, they must be highly porous with large surface/volume ratios to accommodate a large number of cells, they must be biocompatible, i.e., non-toxic to the cells that they carry and to the host tissue into which they are transplanted, they must be capable of promoting cellular interactions, promoting the cells to secrete their own extracellular matrix (ECM) components and allowing the retention of the differentiated function of attached cells.
Polysaccharide matrices, such as for example, alginate scaffolds, have been found to be superior to other scaffolds known in the art such as collagen scaffolds in promoting polarized cell-cell and cell-matrix interactions in cultured hepatocytes. They provide adequate sites for the attachment and growth of a sufficient cell mass to survive and function both in vitro and in vivo; support thick layers of cells, such as cell aggregates; and are capable of maintaining the cells in an active functional state before and after implantation/transplantation into a host tissue, at which time the polysaccharide matrix will also be amenable to vascularization from the surrounding tissue. Polysaccharide matrices do not suffer the drawback of limiting the survival and growth of the cells adjacent to the matrix surface as the cells increase in number within the matrix. Another advantage of polysaccharide matrices is that they are biodegradable but degrade only slowly in vivo and thereby permit the cells carried thereby to become established and to form their own tissue matrix at the site of transplant to the point where they no longer require the polysaccharide matrix.
Although an excellent scaffolding, alginate scaffolds seeded with hepatocytes have limited utility in long-term applications. Although hepatocytes initially demonstrate morphological and functional characteristics after 3 days in culture, including spheroid morphology with enhanced cell-cell interactions, the viability of hepatocytes decline rapidly after 8 days in culture.
Therefore, there remains a need for an artificial liver with the same three-dimensional infra-structure as the native organ comprising viable, differentiated cells, and a method of producing such an artificial liver, which may then be used as a transplant, source of hepatocytes for clinical therapy, and/or for extracorporeal liver assist devices.